1. Field of the Invention
This invention relates to printing/copying apparatus and, more particularly, to printing/copying apparatus utilizing a solid state laser diode to generate an intense beam of coherent light, and specifically relates to a novel, unitary, modular, demountable optical system for producing a precisely defined, accurately located, high definition/resolution moving spot of light from a rotating mirror onto a rotatable photocopy drum.
2. Description of the Prior Art
A number of techniques and apparatus have been suggested for use in laser printing copying. Each has certain characteristics which recommend it for a specific application. Obviously, the goal is to provide a type of printing/copying device which accommodates a variety of purposes and performs these functions effectively and efficiently.
The following patents are considered to be pertinent to the present invention which is considered to be an improvement thereover, as well as an improvement over the earlier filed application U.S. Ser. No. 276,260, as will be described later on herein.
Fleischer, U.S. Pat. No. 3,750,189 shows and describes a helium-neon laser scanner whose light output is coupled through a lens system to a rotating polygon mirror from which the light is reflected through a lens system to a rotating drum. In the Fleischer structure a cylindrical lens is employed to focus the collimated beam of light to a line on a flat facet of the polygon mirror while a second cylindrical or toric lens after the polygon is used to recollimate the light reflected from the polygon. A spherical lens then focusses the recollimated light beam to a small spot on the scan line of the photocopy drum.
For appreciable scan angles it is necessary to use a toroid in order to maintain the spot size over the scan line. However, toroidal lenses are relatively difficult to manufacture and are therefore usually not economically or commercially feasible due to their high cost.
Grafton, U.S. Pat. No. 3,946,150 employs a cylindrical lens near the photo receptor. This proximity requires the use of a long cylindrical lens. Also, since this lens is located close to the developer unit it tends to become coated with toner which degrades the light and resulting photocopy.
Rabedeau, U.S. Pat. No. 4,123,135 is stated to be an improvement over the apparatus described in Fleischer, U.S. Pat. No. 3,750,189. Rabedeau notes that the beam entering the spherical lens need not be collimated. Rabedeau makes use of this by employing a negative cylindrical lens with power in the scan direction following the polygon to produce the same beam divergence in both azimuths for the beam that enters the spherical focussing lenses. The method and structure permits the use of less expensive cylindrical lenses but also tends to flatten the field. However, it remains a very difficult problem to flatten the field over wide angles when relatively high resolution is required.